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What is keratoconus?
Keratoconus is a degenerative eye disease characterized by a progressive weakening of the structural rigidity of the cornea, the transparent membrane located at the front of the eye that acts as the eye’s most powerful natural lens. As the disease progresses, the cornea gradually thins and begins to bulge outward, taking on a distinctive cone-like shape rather than its normal smooth, dome-like curvature.
This condition can affect one or both eyes, although in the vast majority of cases it is bilateral, meaning both eyes are involved, even though the severity can differ significantly between the two. As the cornea becomes progressively more deformed, visual acuity declines correspondingly, meaning that more advanced corneal distortion generally translates into more significant vision impairment.
The good news is that when keratoconus is diagnosed early, through a comprehensive eye examination supported by specific instrumental tests such as corneal tomography, corneal topography, and pachymetry, it is possible to intervene before the disease reaches more advanced and difficult-to-manage stages. Early detection and treatment are therefore absolutely critical to preserving long-term vision and preventing the need for more invasive interventions later in life.
Keratoconus is considered a relatively rare but not uncommon eye disorder, and awareness of its symptoms is essential, particularly for young people, since the condition typically begins during adolescence, a period when subtle changes in vision might otherwise be dismissed as a need for a new eyeglass prescription rather than recognized as a sign of an underlying corneal disease.
What Is the Cornea?
The cornea is a thin, dome-shaped membrane that covers the front surface of the eye. It is the clear, curved tissue through which we can see the iris (the colored part of the eye) and the pupil (the black circular opening at its center that controls how much light enters the eye).
Under normal, healthy conditions, the cornea is completely transparent and contains no blood vessels, relying instead on tears and the aqueous humor (the fluid inside the eye) for nourishment and oxygen. Despite its simple appearance, the cornea is remarkably sophisticated in function: it can be considered the single most powerful lens in the entire visual system. This is because it is the very first structure that light encounters as it travels toward the retina and, ultimately, the brain, and it is responsible for the majority of the eye’s total focusing power, more than the crystalline lens located just behind it.
The cornea works together with the lens to focus incoming light precisely onto the retina, the light-sensitive membrane that lines the back interior surface of the eye. The retina is composed of specialized nerve tissue that captures incoming light signals and converts them into bioelectrical signals. These signals travel along the optic nerve to the brain, where they are interpreted and give rise to the sense of vision.
When the cornea becomes damaged, distorted, or irregularly shaped, as occurs in keratoconus, the light entering the eye is no longer properly focused onto the retina. As a result, images appear blurred, distorted, or doubled, and in the most severe and advanced cases, this progressive damage can eventually lead to significant visual impairment or even complete blindness if left untreated.
What Is Keratoconus?
Keratoconus is a progressive, degenerative eye disease characterized by an abnormal deformation of the cornea. In a healthy eye, the cornea maintains a smooth, rounded, dome-like shape. In someone affected by keratoconus, however, the cornea gradually thins in specific areas and loses its structural integrity, causing it to bulge outward and take on an irregular, cone-like shape rather than its normal spherical curvature.
This structural change fundamentally alters the cornea’s refractive power, meaning its ability to properly bend and focus incoming light rays. As the cornea’s shape becomes increasingly irregular, it can no longer direct light accurately toward the internal structures of the eye. This results in significant visual distortion, with patients typically describing images as blurred, wavy, ghosted, or doubled, symptoms that often worsen progressively over time as the disease advances.
Keratoconus is classified as a corneal ectasia, a broader category of disorders characterized by progressive thinning and bulging of the cornea. It typically affects both eyes, although asymmetrically, with one eye often being more severely affected than the other, particularly in the earlier stages of the disease.
Causes
In most cases, keratoconus first appears between adolescence and early adulthood, typically manifesting sometime between the teenage years and the third decade of life. The disease commonly affects both eyes, although, as mentioned earlier, often to differing degrees between the two.
Current estimates suggest that keratoconus affects at least 1 in every 1,500 people worldwide, although some more recent studies using advanced screening technology suggest the true prevalence may actually be higher, as milder or earlier-stage cases may go undetected using older diagnostic methods. Several factors have been identified that appear to increase an individual’s likelihood of developing this condition:
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Genetic predisposition and family history: Research has demonstrated that approximately 10 to 15 percent of people affected by keratoconus have at least one family member with the same condition, strongly suggesting a hereditary genetic component. Individuals with a first-degree relative (parent or sibling) affected by keratoconus should be particularly vigilant about regular eye examinations.
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Corneal injuries or trauma: Chronic mechanical stress on the cornea can contribute to the development or progression of keratoconus. This includes habitual and vigorous eye rubbing, prolonged or improper use of contact lenses, and long-term irritative processes affecting the surface of the eye. Eye rubbing, in particular, has been strongly linked to keratoconus progression in numerous studies, making patient education about avoiding this habit an important part of disease management.
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Other underlying eye conditions, including:
- Retinitis pigmentosa: an inherited retinal disease that causes a progressive loss of vision, which can advance to complete blindness in the most severe cases.
- Vernal keratoconjunctivitis: a chronic, bilateral, seasonal inflammatory disease affecting the front part of the eye (the conjunctiva and cornea) of unknown cause. This condition primarily affects children and adolescents living in warm or temperate climates and is often associated with intense eye itching that can lead to habitual rubbing.
- Retinopathy of prematurity: a bilateral condition caused by abnormal blood vessel development in the retina, which primarily affects premature infants, particularly those born with very low birth weight.
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Systemic diseases and genetic syndromes, including:
- Down syndrome: a genetic condition caused by the presence of an extra copy (or a partial extra copy) of chromosome 21, which results in intellectual disability and characteristic physical features. Down syndrome has a notably strong association with keratoconus, and individuals with this condition should undergo regular ophthalmologic screening.
- Osteogenesis imperfecta: a genetic disease inherited in an autosomal dominant pattern (meaning affected individuals have at least one affected parent), characterized by abnormalities in collagen synthesis that result in fragile, easily fractured bones and other connective tissue abnormalities, including corneal weakness.
- Ehlers-Danlos syndrome: an inherited connective tissue disorder characterized by joint hypermobility (loose, unstable joints) and unusually elastic, stretchy skin, both of which stem from underlying abnormalities in collagen structure that can also affect the cornea.
- Leber congenital amaurosis: a retinal disease characterized by severe visual impairment or blindness and diminished responses on electrophysiological testing, associated with significant visual impairment appearing within the first year of life.
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Oxidative damage caused by free radicals: Emerging research suggests that the cornea in individuals with keratoconus may be more vulnerable to a specific imbalance in the activity of certain enzymes, specialized proteins that accelerate specific biological reactions within the body. This enzymatic instability may cause certain corneal cells responsible for tissue renewal and repair to function abnormally, leading to progressive thinning of the corneal tissue and the characteristic conical deformity that defines this disease. This oxidative stress theory represents one of the most actively researched areas in understanding the underlying molecular mechanisms of keratoconus.
It is worth noting that keratoconus is generally considered a multifactorial disease, meaning that it likely results from a complex interplay between genetic susceptibility and environmental or mechanical factors, rather than any single isolated cause. This complexity is part of what makes both prediction and prevention challenging for clinicians and researchers alike.
Symptoms
Keratoconus typically follows a progressive course over time, with the disease generally stabilizing over a period of approximately 10 to 20 years from its initial onset. The rate of progression, however, can vary considerably from one patient to another, with some individuals experiencing rapid deterioration while others have a much slower, more gradual course.
In its early stages, keratoconus typically presents with the following characteristic symptoms:
- A general sense of eye irritation or discomfort, sometimes accompanied by mild itching.
- Increased sensitivity to light, a symptom known medically as photophobia.
- Double vision that persists even when one eye is closed (a phenomenon known as monocular diplopia, which is a particularly distinctive early sign of keratoconus).
- Distorted or blurred vision that cannot be fully corrected with standard eyeglasses.
As the disease progresses over months and years, patients typically begin to notice additional and more pronounced symptoms, including:
- Reduced night vision, with increased difficulty seeing clearly in low-light conditions and increased glare or halos around lights, particularly while driving at night.
- A progressive increase in myopia (nearsightedness) or the development of irregular astigmatism specifically caused by keratoconus, which often requires frequent changes to eyeglass prescriptions.
- Increasing difficulty wearing standard, off-the-shelf contact lenses, since the irregular corneal shape no longer allows for a proper, comfortable fit.
- The development of corneal edema (swelling) and, in more advanced cases, corneal scarring, which can further compromise vision quality.
- Growing difficulty performing everyday tasks that rely on clear vision, such as driving, watching television, reading a book, or recognizing faces from a distance.
The speed at which these symptoms develop and progress can vary significantly from person to person. In cases where the disease is allowed to advance without treatment, it can eventually lead to severe and near-total impairment of vision, along with structural compromise of the integrity of the eyeball itself, a situation that can sometimes require emergency surgical intervention.
It is important for patients, and particularly for parents of adolescents experiencing unexplained vision changes, to seek prompt evaluation by an eye care professional whenever these warning signs appear, since early intervention offers the best chance of halting disease progression and preserving long-term visual function.
Diagnosis
The diagnosis of keratoconus is established during a comprehensive eye examination performed by an ophthalmologist or optometrist. If keratoconus is suspected based on the initial examination and the patient’s reported symptoms, additional specialized diagnostic tests are typically ordered to confirm the presence of the disease with certainty. Establishing an accurate and timely diagnosis is essential in order to prevent further corneal damage and, in the most severe cases, avoid complete and irreversible vision loss.
The most useful and commonly employed diagnostic tests include:
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Corneal topography: This test creates a detailed map of the surface curvature of the cornea. It is performed using a computerized optical instrument that projects light patterns onto the surface of the cornea and analyzes the reflected pattern to assess its shape, curvature, and any areas of irregular thinning or bulging. Corneal topography is often considered the gold standard for detecting the earliest, subtlest signs of keratoconus, sometimes even before the patient notices any significant symptoms.
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Corneal tomography: This more advanced instrumental examination provides a three-dimensional analysis of the cornea, studying its curvature, overall shape, and thickness at multiple different levels and locations across its entire surface, both the front and back layers. This comprehensive assessment allows clinicians to detect even very early-stage keratoconus that might not yet be apparent through topography alone.
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Pachymetry: This test measures the precise thickness of the cornea at multiple points, allowing the clinician to identify the thinnest point of the cornea, which is often the area most affected by the disease process and the location where the characteristic conical bulging tends to occur first.
In addition to these primary diagnostic tools, an ophthalmologist may also perform a slit-lamp examination to look for specific physical signs associated with keratoconus, such as Vogt’s striae (fine vertical stress lines within the corneal tissue) or Fleischer’s ring (a characteristic pigmented ring that can sometimes be seen surrounding the base of the cone). A refraction test to measure visual acuity and the degree of astigmatism or myopia present is also typically performed as part of the overall diagnostic workup.
Treatment
The most appropriate treatment approach for keratoconus is selected based on the individual patient’s specific condition and the stage of disease progression at the time of diagnosis. The main therapeutic strategies currently available include:
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Eyeglasses or specialized contact lenses: In the earliest stages of keratoconus, vision can often be adequately corrected using standard eyeglasses or, more commonly, specially designed rigid gas-permeable or scleral contact lenses. These specialized lenses help compensate for the irregular corneal surface by creating a smooth, uniform refractive surface, thereby improving visual acuity even though they do not stop the underlying disease progression.
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Corneal cross-linking (CXL): This treatment is primarily used in the earlier stages of the disease, when the goal is to halt or slow further progression rather than reverse existing damage. The procedure involves instilling eye drops containing vitamin B2, also known as riboflavin, which penetrates into the corneal tissue. The cornea is then exposed to a carefully controlled, low-dose irradiation with type A ultraviolet light (UVA) for approximately 30 minutes, during which the riboflavin drops are reapplied every 5 minutes to maintain adequate saturation of the corneal tissue. This combination of riboflavin and UVA light triggers the formation of new molecular bonds, known as cross-links, between collagen fibers within the cornea, effectively strengthening and stiffening the corneal structure to prevent further thinning and bulging. Following the procedure, the eye is treated with antibiotic eye drops and ointments and is typically protected with a bandage or a therapeutic contact lens, which is generally worn for approximately 3 to 4 days to promote healing and reduce discomfort. Corneal cross-linking is currently considered the only treatment capable of actually halting the progression of keratoconus, rather than simply managing its symptoms.
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Intrastromal corneal ring segments and phakic lens treatment: This approach involves the surgical insertion of small, curved plastic ring segments, known as intrastromal rings, directly into the layers of the cornea. The purpose of these rings is to help stabilize the cornea’s shape and slow the progression of the disease by physically reducing the abnormal curvature. This procedure has the added benefit of often improving visual acuity by flattening the irregular corneal surface. In some cases, once the cornea has been stabilized through this method, an additional procedure may be performed to implant phakic lenses (essentially a permanent artificial lens implanted in front of the eye’s natural lens) to correct any remaining visual defects, such as myopia, astigmatism, or hyperopia. This combined approach is particularly recommended for patients who, due to the severity of their keratoconus, are no longer able to comfortably or successfully wear standard contact lenses.
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Corneal transplantation (keratoplasty): When keratoconus has progressed to an advanced stage where vision can no longer be adequately corrected using contact lenses or the less invasive surgical procedures described above, a corneal transplant may become necessary. This procedure involves surgically removing the diseased, deformed cornea and replacing it with a healthy, clear cornea obtained from a deceased donor. Depending on the extent of corneal damage, surgeons may perform either a full-thickness transplant (penetrating keratoplasty) or a partial-thickness transplant that preserves some of the patient’s own healthy corneal layers (deep anterior lamellar keratoplasty, or DALK). Corneal transplantation is generally considered a treatment of last resort, reserved for the most advanced and severe cases of keratoconus, but it has a generally favorable success rate and can significantly restore vision in appropriately selected patients.
The choice among these treatment options depends heavily on multiple factors, including the patient’s age, the specific stage and rate of progression of the disease, the degree of visual impairment already present, and the overall health and thickness of the remaining corneal tissue. A close, ongoing partnership between the patient and their ophthalmologist is essential to determine the most appropriate treatment path and to monitor the disease over time, since keratoconus often requires long-term follow-up even after successful treatment.
Prevention
Unfortunately, there are currently no proven strategies capable of truly preventing the onset of keratoconus, since its exact underlying causes remain incompletely understood and likely involve a complex combination of genetic and environmental factors that are difficult to modify. However, undergoing frequent and regular eye examinations, particularly for individuals with a known family history of keratoconus or those experiencing frequent, sudden, or unexplained changes in their vision, can play an extremely valuable role in achieving an early diagnosis.
Early detection is truly the single most important factor in preventing the establishment of irreversible corneal damage, since it allows for the prompt initiation of appropriate treatment, such as corneal cross-linking, before the disease has the opportunity to progress to more advanced and difficult-to-manage stages. For this reason, ophthalmologists generally recommend that individuals with any of the risk factors discussed earlier in this article, such as a family history of the condition, an associated systemic disease, or a habit of frequent and vigorous eye rubbing, undergo more frequent eye examinations than the general population.
Additionally, patients who have already been diagnosed with keratoconus, or who are at elevated risk due to family history, are strongly encouraged to avoid habitually rubbing their eyes, since this mechanical stress has been consistently linked to disease progression in multiple scientific studies. Patients with underlying allergic conditions that cause eye itching, such as allergic conjunctivitis, should work closely with their doctor to manage these symptoms effectively, thereby reducing the urge to rub their eyes and potentially lowering their long-term risk of keratoconus progression.
Frequently Asked Questions
Is keratoconus a hereditary condition?
Keratoconus does have a genetic component, with approximately 10 to 15 percent of affected individuals having a family member with the same condition. However, it is considered a multifactorial disease, meaning that genetic predisposition alone is generally not sufficient to cause the disease; environmental and mechanical factors also appear to play an important contributing role.
Can keratoconus lead to complete blindness?
While keratoconus can cause significant visual impairment if left untreated, complete blindness is relatively rare with modern diagnostic and treatment options. Early detection and appropriate treatment, such as corneal cross-linking, can effectively halt disease progression in most patients, and even in advanced cases, corneal transplantation offers a highly effective option for restoring vision.
At what age does keratoconus typically develop?
Keratoconus most commonly begins during adolescence or early adulthood, typically between the ages of 10 and 25, although it can occasionally develop earlier or later in life.
Can I still wear contact lenses if I have keratoconus?
Many patients with keratoconus can successfully wear specialized contact lenses, such as rigid gas-permeable or scleral lenses, which are specifically designed to accommodate the irregular corneal shape. However, as the disease progresses, some patients may find it increasingly difficult to achieve a comfortable or effective fit with any type of contact lens.
Is corneal cross-linking a permanent cure for keratoconus?
Corneal cross-linking is not a cure in the traditional sense, but it is highly effective at halting or significantly slowing the progression of the disease when performed at an appropriate stage. It does not reverse existing corneal damage or restore the cornea to its original shape, which is why early diagnosis and treatment are so critical.
Sources and Bibliography
- Fournié P, Touboul D, Arné J-L, Colin J, Malecaze F. “Keratoconus”. J Fr Ophtalmol. 2013 Sep;36(7):618-26.
- Imbornoni LM, McGhee CNJ, Belin MW. “Evolution of Keratoconus: From Diagnosis to Therapeutics”. Klin Monbl Augenheilkd. 2018 Jun;235(6):680-688.
- Davidson AE, Hayes S, Hardcastle AJ, Tuft SJ. “The Pathogenesis of Keratoconus”. Eye (Lond). 2014 Feb;28(2):189-95.
